In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added gaseous products from lower-fuel-value carbonaceous feedstocks, such as petroleum coke, coal and biomass, is receiving renewed attention. The catalytic gasification of such materials to produce methane and other value-added gases is disclosed, for example, in U.S. Pat. Nos. 3,828,474, 3,998,607, 4,057,512, 4,092,125, 4,094,650, 4,204,843, 4,468,231, 4,500,323, 4,541,841, 4,551,155, 4,558,027, 4,606,105, 4,617,027, 4,609,456, 5,017,282, 5,055,181, 6,187,465, 6,790,430, 6,894,183, 6,955,695, US2003/0167961A1, US2006/0265953A1, US2007/000177A1, US2007/083072A1, US2007/0277437A1 and GB1599932.
In general, carbonaceous materials, such as coal, biomass and/or petroleum coke, can be converted to a plurality of gases, including value-added gases such as methane, by the reaction of the material in the presence of a catalyst source and steam at elevated temperatures and pressures. Fine unreacted carbonaceous materials are removed from the raw gas product, and the gases are cooled and scrubbed in multiple processes to remove undesirable contaminants and other side-products including carbon monoxide, hydrogen, carbon dioxide and hydrogen sulfide, to produce a methane product stream.
The hydromethanation of a carbon source to methane typically involves four separate reactions:Steam carbon: C+H2O→CO+H2  (I)Water-gas shift: CO+H2O→H2+CO2  (II)CO Methanation: CO+3H2→CH4+H2O  (III)Hydro-gasification: 2H2+C→CH4  (IV)
In the hydromethanation reaction, the result is a “direct” methane-enriched raw product gas stream, which can be subsequently purified and further methane-enriched to provide the final methane product. This is distinct from conventional gasification processes, such as those based on partial combustion/oxidation of a carbon source, where a syngas (carbon monoxide+hydrogen) is the primary product (little or no methane is directly produced), which can then be further processed to produce methane (via catalytic methanation, see reaction (III)) or any number of other higher hydrocarbon products. When methane is the desired end-product, the hydromethanation reaction provides the possibility for increased efficiency and lower methane cost than traditional gasification processes.
In the hydromethanation reaction, the first three reactions (I-III) predominate to result in the following overall reaction:2C+2H2O→CH4+CO2  (V).
The overall reaction is essentially thermally balanced; however, due to process heat losses and other energy requirements (such as required for evaporation of moisture entering the reactor with the feedstock), some heat must be added to maintain the thermal balance.
The reactions are also essentially syngas (hydrogen and carbon monoxide) balanced (syngas is produced and consumed); therefore, as carbon monoxide and hydrogen are withdrawn with the product gases, carbon monoxide and hydrogen need to be added to the reaction as required to avoid a deficiency.
In order to maintain the net heat of reaction as close to neutral as possible (only slightly exothermic or endothermic), and maintain the syngas balance, a superheated gas stream of steam, carbon monoxide and hydrogen is often fed to the hydromethanation reactor. Frequently, the carbon monoxide and hydrogen streams are recycle streams separated from the product gas, and/or are provided by reforming a portion of the product methane. See, for example, U.S. Pat. Nos. 4,094,650, 6,955,595 and US2007/083072A1.
Gas recycle loops generally require at least additional heating elements (fired superheaters) and pressurization elements to bring the recycle gas stream to a temperature and pressure suitable for introduction into the hydromethanation reactor. Further, the separation of the recycle gases from the methane product, for example by cryogenic distillation, and the reforming of the methane product, greatly increase the engineering complexity and overall cost of producing methane, and decrease the overall system efficiency.
Steam generation is another area that can increase the engineering complexity of the overall system. The use of externally fired boilers, for example, can greatly decrease overall system efficiency.
Therefore, a need remains for improved hydromethanation processes where gas recycle loops are minimized and/or eliminated, and steam is generated efficiently, to decrease the complexity and cost of producing methane.